1. Field of the Invention
This invention relates to an optical apparatus for measuring the dimensions of a minute object such as a minute pattern on the surface of a photomask for the manufacture of integrated circuits.
2. Description of the Prior Art
Measurement by visual observation using a microscope has heretofore been practised to measure the dimensions of a pattern or the like formed on a photomask for the manufacture of IC, but as the requirement for higher accuracy and higher reproducibility of measurement becomes more critical, such measurement is being replaced by photoelectric measurement or measurement by image processing using ITV.
As a kind of photoelectric measurement, there is a slit scan method, and it is disclosed in detail, for example, in Japanese Patent Publication No. 23172/1978 by the assignee of the present invention. According to this, an enlarged image of a minute object by an objective lens of high magnification is formed on a slidable mirror having a slit, and by moving this mirror relative to the image, an intensity distribution created by the contrast of the image of the light passed through the slit is found, and the dimensions of the minute object can be found from this light intensity distribution. In this apparatus, the reflected light from the slidable mirror is condensed by a relay lens and the object image is reimaged and observed through an eyepiece and therefore, the entire measurement field can be observed and the in-focus state and position of the image on the mirror can be observed.
In such a conventional apparatus, however, the object image is re-imaged by the reflected light from the slidable mirror and therefore, the light that can be condensed by the relay lens is insufficient and the observed image is liable to be small in field of view and dark.